The field of biosensors is an active research area. A sensor probe can be dipped into a biological fluid to measure the presence and/or concentration of an analyte, such as protein, a particular molecule, or a group of molecules. Biosensors generally have two principal components, a molecular-recognition element and a transducing or signal-generating element. Two common problems associated with biomedical sensing technology are the need for high specificity and the susceptibility of the sensing devices to fouling. Moreover, many of the current sensing devices are designed to detect only one or a small number of analytes or physiological conditions.
The molecular-recognition element is often not specific enough for the particular molecule or group of molecules (analytes) of interest and the molecular-recognition element can often cross-react with other molecules, causing a detection error.
Biofouling is the nonspecific adsorption and adhesion of biomolecules to a surface. When a biosensor is contacted with a biological fluid, biofouling is inevitable. In some sensing configurations, the biofouling is severe enough to render the device inoperable. Therefore, there is a need in the art for a signal-generating surface that, rather than trying to prevent biofouling, takes a reading based on the amount of nonspecific biomolecule adsorption, such as protein adsorption.
Biosensors have used a variety of detection devices in an attempt to quantitate the signal produced from the signal-generating system and the molecular-recognition element.
A common molecular-recognition element is an antibody, preferably a monoclonal antibody. In principle, antibodies are ideal candidates to use as molecular-recognition elements in biosensor design. Antibodies have the ability to bind antigens quite selectively and with binding constants (which indicate the ability of an antigen to interact with an antibody) that are neither too high nor too low. Antibodies can now be raised to react and bind specifically to numerous biomolecules, drugs, viruses, and cellular materials. However, because of the relatively high molecular weight of antibodies as compared with antigens, it is often difficult to couple an antibody-antigen binding reaction to a transducer in such a manner that the observed signal reflects an antibody-antigen interaction in a quantitative manner. Much of the biosensor art involves optimizing the union of the molecular recognition elements with the transducing or signal-generating elements.
One approach has been to coat piezoelectric crystals with antibodies to make biosensors for gaseous pollutants, such as the pesticide parathion. In the case of parathion, anti-parathion antibodies are coated on quartz piezoelectric crystals using bovine serum albumin/glutaraldehyde for immobilization. When mounted on a suitable apparatus, the piezoelectric crystals undergo changes in frequency if exposed to the antigen parathion. Such a biosensor may be sensitive in the parts-per-billion range.
It is also possible to use a fiber optic immunosensor with antibodies coating a fiber optic cable and detection by means of internal reflection spectroscopy. The interaction of the antibody coated on the fiber optic cable with its antigen can be monitored optically on a microscale. Such a biosensor has been used to measure concentrations of the drug methotrexate.
Immunoreaction biosensors have been coupled to electrochemical transducers. The antibodies are immobilized on a cellulose acetate membrane, and potential changes occur when the antigen-positive serum is added to the sample.
Piezoelectric systems are based upon a variation in the propagation speed of acoustic waves at the surface or in the bulk of a piezoelectric material, such as a quartz crystal. The variation is due to mass changes in the biomolecules bound to the coated layer. Immunological systems based upon a monoclonal IgG system have used a SAW (surface acoustic wave) technique. Results have been obtained with a detection limit as low as 1 ng. However, such measurements have suffered from buffer influence, drift, and calibration difficulties.
Another type of sensor measures the changes in capacitance due to changes in the dielectric constant caused by antibody-antigen interaction. An example of a biosensor consists of interdigitated copper electrodes on a glass surface, and insulated by a layer of parylene, and covered by a silicon monoxide film. An aminosilane allows a hapten to be fixed on the surface of the silicon monoxide. The addition of a solution containing antibodies induces a decrease in the capacitance. This is because of the variation of the dielectric constant under the membrane due to the binding of antibodies to the surface-bound antigen (hapten). Thus, the binding of the antigen or the antibody induces a variation of the heterostructure capacitance. Any variation of the surface potential leads to a shift in the capacitance-versus-voltage curve in the inversion range. The increase in the thickness of the dielectric layer induces a capacitance decrease in the accumulation range, which can be directly related to the size of the immobilized biomolecules and to the quantity of the titrated antigen.
Outside the field of biosensors, specific chemical sensors have been used to detect specific chemicals using pattern-recognition analysis of data from a sensor array. A chemical sensor array has sensors coated with different absorptive chemicals. The sensitivity and specificity of each of the absorption surfaces may vary. The data are collected in several channels of unique information provided by the array. The pattern recognition results recognize groups of chemicals through uniqueness of the patterns. Pattern recognition, as applied to a chemical sensor, requires: 1) that the analyte and the instrument's response are related; 2) that the analyte can be adequately represented as a set of sensor responses; 3) that a relationship can be discovered between various analytes and their responses by applying pattern-recognition methods; and 4) that the relationship can be extrapolated to other analytes in similar classes. There is a need in the art to use pattern recognition techniques in the field of biosensors, and especially for biosensors that have non-specific interactions.
In summary, the field of biosensors has focused on the ability to increase the specificity of the sensor and its sensitivity to the analyte. Both goals are difficult to achieve in a biological fluid. Accordingly, there is a need in the art for a sensor-type device which tries not to achieve selectivity or sensitivity, but instead can identify a variety of nonspecific molecules or physiological conditions while not requiring high specificity.